Communication systems employing multicarrier modulation have become increasingly popular. A common multicarrier modulation, discrete multitone modulation (DMT), uses the Fourier transform for modulation and demodulation. DMT assigns different numbers of bits to different subchannels depending on the subchannel signal to noise ratio (SNR), which allows it to approach capacity for frequency selective channels. A related multicarrier modulation, orthogonal frequency division multiplexing (OFDM), assigns a fixed number of bits to all subchannels. Examples of wireline systems using DMT include asymmetric digital subscriber lines (ADSL) and one of the proposed very high speed digital subscriber line (VDSL) standards. OFDM appears in wireless systems such as digital audio and video broadcasting and wireless networking.
One of the reasons for the popularity of DMT is the existence of simple equalization schemes to compensate for frequency selective or multipath channels based on the use of a cyclic prefix. At the DMT transmitter the input is segmented into blocks, the inverse discrete Fourier transform (IDFT) is applied, and a cyclic prefix is added. At the receiver the cyclic prefix is removed and the discrete Fourier transform (DFT) of the data is taken. Fast Fourier transform (FFT) and inverse fast Fourier transform (IFFT) algorithms are typically used to implement the DFT and IDFT, respectively. If the channel memory (including transmit and receive filters) is less than or equal to the length of the cyclic prefix (or has been shortened to that length by a time-domain equalizer (TEQ)), then the frequency selective channel is effectively divided into parallel flat fading subchannels. Equalization is then possible for the subchannels by multiplying each of them by a complex number (known as the frequency-domain equalizer (FEQ)).
For most standards which use DMT the length of the cyclic prefix is fixed. The length is determined based on some typical or worst case channel assumption. However, in practice, many observed channels will be shorter than (or can be shortened to a length less than) the cyclic prefix.
Narrowband interferers such as radio frequency interference (RFI) can result in noise which is strongly correlated between subchannels. RFI significantly degrades system performance, since noise which is not periodic in the subchannel spacing gets spread to many subchannels. This is because the selection of the data and discarding of the prefix is equivalent to multiplying the received signal (including noise) by a rectangular window. Multiplication in time is equivalent to convolution in the frequency-domain with a sinc-like function. Since the sidelobes of the sinc are relatively high and decay rather slowly, narrowband interference can effect a large number of subchannels.
Receiver windowing is a technique which exploits redundant information in the cyclic prefix to improve the SNR of the equalized signal. As its name suggests, receiver windowing is a receiver-only technique, so it is applicable to systems without requiring a change to the transmitter (which is typically the specified part of a standard). Receiver windowing is well suited to cases where the channel is shorter than the prefix, and there is strong correlation in the noise. It works by using samples from the cyclic prefix to construct a window which effects the noise component of the received signal, but leaves the data component of the received signal alone. The result is that the noise is convolved in frequency with a window which has lower sidelobes than the sinc-like function, so it spreads less to neighboring subchannels. The low complexity of receiver windowing fits in well with the rest of the DMT receiver structure.
A number of variations on the receiver windowing operation have been proposed in the literature. Fixed windows (typically based on Hamming, Hann or raised cosine shapes) have been proposed. The use of a DFE to remove structured subchannel interference which resulted from a long window has been proposed. The combination of per tone equalization and receiver windowing was considered, and it was shown that windowing effectively lengthens the per tone equalizers (i.e., receiver windowing is a subset of per tone equalization). In essence, per tone equalization computes a window and equalizer for each subchannel.